1. Field of the Invention
The present invention relates to a method of detecting a failure of a humidity sensor that is disposed in the vicinity of a hydrocarbon adsorbent in the exhaust system of an internal combustion engine.
2. Description of the Related Art
Some known exhaust systems of internal combustion engines have an exhaust gas purifier disposed in the exhaust passage, which may comprise a hydrocarbon adsorbent such as zeolite or a hydrocarbon adsorption catalyst comprising a composite combination of a hydrocarbon adsorbent and a three-way catalyst, for adsorbing hydrocarbons (HC) in the exhaust gas while the catalytic converter such as a three-way catalyst or the like is not functioning sufficiently, i.e., while the catalytic converter is not sufficiently high in temperature and not activated as when the internal combustion engine starts to operate at a low temperature. The hydrocarbon adsorbent has a function to adsorb hydrocarbons in the exhaust gas at relatively low temperatures below 100xc2x0 C., for example, and operates to release the adsorbed hydrocarbons when heated to a certain temperature in the range from 100 to 250xc2x0 C., for example.
The applicant of the present application has attempted to evaluate a deteriorated state of the hydrocarbon adsorbent based on the output of a humidity sensor, which is disposed in the vicinity of the hydrocarbon adsorbent, e.g., downstream of the hydrocarbon adsorbent, under a certain operating condition of the internal combustion engine such as when the internal combustion engine is idling immediately after it has been started (see, for example, Japanese laid-open patent publication No. 2001-323811). The above evaluating technique is based on the fact that the hydrocarbon adsorbent has a property of adsorbing not only HC but also water contained in the exhaust gases emitted from the internal combustion engine, and the HC adsorbing ability (the maximum amount of HC that can be adsorbed) and the water adsorbing ability (the maximum amount of water that can be adsorbed) of the hydrocarbon adsorbent are highly correlated to each other. According to this evaluating process, the amount of water adsorbed by the hydrocarbon adsorbent is recognized based on the output of the humidity sensor, and the deteriorated state of the hydrocarbon adsorbent is evaluated based on the recognized amount of water adsorbed by the hydrocarbon adsorbent.
However, the above evaluating process fails to evaluate the deteriorated state of the hydrocarbon adsorbent in the event that the humidity sensor fails to operate. Therefore, it is desirous to have a technique on hand for detected whether or not the humidity sensor has suffered a failure i.e., whether or not the humidity sensor is normal.
One proposal is to detect whether or not the humidity sensor has suffered a failure based on the output of the humidity sensor in a certain operating state of the internal combustion engine. However, the internal combustion engines on automobiles operate in various operating states which tend to fluctuate with time or vary under various conditions. The internal combustion engines may not necessarily operate reliably in a certain operating state in which it is possible to detect whether or not the humidity sensor has suffered a failure. As the operating states of the internal combustion engines are liable to fluctuate with time, the output of the humidity sensor is subject to variations in the responsiveness thereof. It is thus generally difficult to detect whether or not the humidity sensor has suffered a failure in a certain operating state of the internal combustion engine.
It is therefore an object of the present invention to provide a method of detecting, stably and easily, a failure of a humidity sensor disposed in the vicinity of a hydrocarbon adsorbent in the exhaust system of an internal combustion engine.
According to the present invention, the humidity in the exhaust passage of an internal combustion engine near a hydrocarbon adsorbent disposed therein changes after the internal combustion engine is shut off, as follows: When the internal combustion engine is shut off after it has been normally operated continuously for a certain period of time, the humidity of the exhaust gas decreases as the hydrocarbon adsorbent adsorbs moisture in an exhaust gas remaining thereafter after the temperature of the hydrocarbon adsorbent has dropped to a temperature capable of adsorbing the moisture. When the adsorption of the moisture by the hydrocarbon adsorbent is saturated, the humidity near (around) the hydrocarbon adsorbent becomes substantially constant, or more specifically, its time-dependent changes are extremely small. The humidity which has become substantially constant near the hydrocarbon adsorbent changes depending on the deteriorated state of the hydrocarbon adsorbent, i.e., the humidity is higher as the deterioration of the hydrocarbon adsorbent progresses. If the humidity sensor is normal, however, the humidity falls in a certain range. Finally, because a gas exchange progresses between the exhaust passage and its exterior (generally, the gas exchange progresses very slowly), the humidity in the exhaust passage is converged to a humidity equivalent to the humidity outside of the exhaust passage.
A method of detecting a failure of a humidity sensor according to the present invention has been devised in view of the above phenomenon. According to the present invention, there is provided a method of detecting a failure of a humidity sensor in an exhaust system having a hydrocarbon adsorbent disposed in an exhaust passage of an internal combustion engine for adsorbing hydrocarbons contained in an exhaust gas emitted from the internal combustion engine, the humidity sensor being disposed in the exhaust passage for detecting a humidity in the exhaust passage near the hydrocarbon adsorbent, the method comprising the step of detecting whether or not the humidity sensor has failed based on an output signal of the humidity sensor within a predetermined period in which the output signal of the humidity sensor is substantially constant and a humidity represented by the output signal of the humidity sensor is not converged to a humidity outside of the exhaust passage, after the internal combustion engine is shut off.
The output signal from the humidity sensor in the predetermined period, which is used in detecting whether or not the humidity sensor has failed, corresponds to the humidity (detected humidity) near the hydrocarbon adsorbent when the humidity near (around) the hydrocarbon adsorbent is substantially constant and in a state before the humidity near (around) the hydrocarbon adsorbent is converted to the humidity outside of the exhaust passage after the internal combustion engine is shut off (such a state will hereafter be referred to occasionally as xe2x80x9csteady humidity statexe2x80x9d). Because the hydrocarbon adsorbent and the state of the exhaust gas around the humidity sensor near the hydrocarbon adsorbent are substantially static (steady) in the predetermined period as the internal combustion engine is shut off, the output signal from the humidity sensor in the predetermined period is highly stable. Because the output signal from the humidity sensor in the predetermined period depends on the steady humidity, it is not affected by variations in the responsiveness of the humidity sensor. Accordingly, it is possible to detect stably and easily whether or not the humidity sensor has failed based on the output signal from the humidity sensor within the predetermined period.
According to the present invention, the predetermined period comprises, for example, a period after a lapse of a predetermined time at least after the internal combustion engine is shut off. Specifically, until the hydrocarbon adsorbent adsorbs moisture therearound and is saturated, i.e., until the steady humidity state is initiated, after the internal combustion engine is shut off, a certain period of time (two to four hours for the exhaust system of an automobile) is needed. According to the present invention, it is detected whether or not the humidity sensor has failed based on the output signal from the humidity sensor after elapse of the predetermined time. This allows the output signal from the humidity sensor, which is appropriate for detecting whether or not the humidity sensor has failed, to be easily obtained without the need for monitoring the output signal from the humidity sensor frequently.
According to the present invention, the predetermined period comprises a period after at least a temperature close to the hydrocarbon adsorbent is substantially equal to a temperature outside of the exhaust passage. Specifically, after the internal combustion engine is shut off, the temperature near the hydrocarbon adsorbent drops and is finally converged to a temperature substantially equal to the temperature outside of the exhaust passage. When the temperature near the hydrocarbon adsorbent is substantially equal to the temperature outside of the exhaust passage, because the saturated water vapor pressure near the hydrocarbon adsorbent is substantially constant, the humidity near the hydrocarbon adsorbent is also substantially constant. Accordingly, if the predetermined period comprises the period after the temperature close to the hydrocarbon adsorbent is substantially equal to the temperature outside of the exhaust passage, then it is possible to obtain the output signal of the humidity sensor which is appropriate for detecting whether or not the humidity sensor has failed.
According to the present invention, the predetermined period terminates when a predetermined time elapses after the internal combustion engine is shut off. Specifically, after the internal combustion engine is shut off, the humidity near the hydrocarbon adsorbent is finally converged to the humidity substantially equal to the humidity outside of the exhaust passage, as described above. Consequently, the predetermined period is selected to terminate when the predetermined time elapses after the internal combustion engine is shut off, and it is detected whether or not the humidity sensor has failed based on the output signal from the humidity sensor prior to the time when the predetermined time elapses. It is thus possible to obtain the output signal of the humidity sensor which is appropriate for detecting whether or not the humidity sensor has failed. The predetermined time referred to above is a time (24 to 72 hours for the exhaust system of an automobile) sufficiently longer than the time when the steady humidity state is initiated after the internal combustion engine is shut off.
According to the present invention, it is detected whether or not the humidity sensor has failed based on the output signal from the humidity sensor after the internal combustion engine is shut off at least under predetermined operating conditions. This allows a state of the exhaust gas (the humidity of the exhaust gas, or the like) in the exhaust passage after the internal combustion engine is shut off to be kept optimum in detecting whether or not the humidity sensor has failed. Therefore, the reliability of the process of detecting whether or not the humidity sensor has failed can be increased.
More specifically, the predetermined operating conditions should preferably include a condition relative to an air-fuel ratio before the internal combustion engine is shut off, such that it should be detected whether or not the humidity sensor has failed based on the output signal from the humidity sensor if the air-fuel ratio at least immediately before the internal combustion engine is shut off is kept at an air-fuel ratio close to a stoichiometric air-fuel ratio continuously for a predetermined period of time.
Specifically, if the internal combustion engine is operated with the air-fuel ratio thereof being kept as an air-fuel ratio close to the stoichiometric air-fuel ratio, then the exhaust gas emitted from the internal combustion engine contains a relatively large amount of moisture, and the moisture contained in the exhaust gas has a substantially constant concentration. If, therefore, the air-fuel ratio immediately before the internal combustion engine is shut off is continuously kept as an air-fuel ratio close to the stoichiometric air-fuel ratio, then an exhaust gas containing a sufficient amount of moisture at a substantially constant concentration is present near the hydrocarbon adsorbent immediately after the internal combustion engine is shut off. Therefore, the hydrocarbon adsorbent smoothly adsorbs moisture and is saturated thereby, so that the humidity near the hydrocarbon adsorbent reliably enters the steady humidity state, and the stability of the humidity of the hydrocarbon adsorbent in the steady humidity state is increased. As a result, the output signal from the humidity sensor in the predetermined period when the humidity sensor has failed to operate and the output signal from the humidity sensor in the predetermined period when the humidity sensor has not failed to operate are clearly distinguished from each other, thereby increasing the accuracy of the process of detecting whether the humidity sensor has failed or not.
Preferably, the predetermined operating conditions include a condition relative to a warmed-up state before the internal combustion engine is shut off, such that the step of detecting whether or not the humidity sensor has failed is carried out if an engine temperature at least immediately before the internal combustion engine is shut off is equal to or greater than a predetermined temperature. According to such a process, when the engine temperature is equal to or greater than the predetermined temperature, the internal combustion engine has been warmed up sufficiently, i.e., the air-fuel mixture is combusted stably in the internal combustion engine, and the hydrocarbon adsorbent has been heated sufficiently to release the moisture that has been adsorbed by the hydrocarbon adsorbent. When the internal combustion engine is shut off in such a state, any variations in the humidity of the exhaust gas in the hydrocarbon adsorbent immediately after the internal combustion engine is shut off are reduced, and the hydrocarbon adsorbent smoothly adsorbs a maximum amount of moisture depending on the deteriorated state thereof after the temperature of the hydrocarbon adsorbent is lowered to a certain extent. As a consequence, the stability of the output signal of the humidity sensor in the predetermined period is increased, resulting in an increase in the accuracy with which to detect whether or not the humidity sensor has failed.
According to the present invention, the step of detecting whether or not the humidity sensor has failed should preferably be carried out by comparing the output signal of the humidity sensor acquired within the predetermined period with a threshold established depending on the temperature of the hydrocarbon adsorbent when data of the output signal of the humidity sensor is acquired. As described above, the maximum amount of moisture that can be adsorbed by the hydrocarbon adsorbent is affected by the temperature of the hydrocarbon adsorbent. Basically, the maximum amount of moisture that can be adsorbed by the hydrocarbon adsorbent is greater as the temperature of the hydrocarbon adsorbent is lower. Therefore, the output signal of the humidity sensor in the predetermined period is affected by the temperature of the hydrocarbon adsorbent. The threshold to be compared with the output signal from the humidity sensor within the predetermined period for detecting whether or not the humidity sensor has failed is set depending on the temperature of the hydrocarbon adsorbent at the time data of the output signal from the humidity sensor is acquired. Therefore, the reliability of the process of detecting whether or not the humidity sensor has failed can be increased.
It is preferable to provide two thresholds, i.e., an upper limit threshold and a lower limit threshold, as the above threshold. If the output signal from the humidity sensor in the predetermined period falls in a range between the upper limit threshold and the lower limit threshold, then it is judged that the humidity sensor does not suffer a failure. If the output signal from the humidity sensor falls out of the above range, then it is judged that the humidity sensor has failed to operate.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.